In a variety of applications, the ability to shed water or other contaminants is important, and there have been developed hydrophobic surfaces designed to reduce friction to the flow or retention of water or other liquid on the surface. Hydrophobic materials have surfaces that are difficult to wet with water or ice, with water contact angles generally in excess of 120°. Superhydrophobic surfaces generally have contact angles of 150° or more. Hydrophobic materials are characterized by Cassie's law which describes the effective contact angle θc for a liquid on the surface. Cassie's law explains how roughing up a surface increases the apparent surface angle between a liquid and the surface. The surface energy of the hydrophobic surface is directly related to its ability to repel water. As surface energy decreases water droplets have increased preference to cling to themselves as opposed to the surface. It has been found that the external surfaces of many plants and animals have a rough surface structure combined with an ideal surface chemistry to create self-cleaning, water-repellant surfaces. For example, the self-cleaning characteristics found on the leaf surface of the N. nucifera (the white lotus) and the wing surface of many insects combine a topology describing a high degree of surface roughness with a chemistry that exhibits=low surface energy thus creating a superhydrophobic surface such that it sheds liquids of various types allowing particulates to be removed when subjected to an external force such as rolling water droplet, or flowing air. Superhydrophobic coatings utilizing nano sized irregularities applied to a surface form a high contact angle which resist wetting and adherence of dirt and contaminants.
For example, in association with structures such as aircraft or aerospace exterior surfaces, the surfaces of heat exchange equipment, and many others, are susceptible to the buildup of ice, water, and other contaminants that can interfere with the operation of such surfaces or reduce their efficiency. For example, the buildup of ice, water, and/or other contaminants on aircraft wings, propellers, rotors, and other functional surfaces can interfere with or degrade the operating performance of the aircraft, heat exchanger equipment or the like. When such buildups occur, much time and cost can be expended in the removal thereof. To prevent or mitigate such buildup, hydrophobic surfaces, which tend to repel water, may be utilized.
In other applications, such as water or other liquid transport conduits, microfluidic devices or the like, resistance to flow of the liquid can be imposed by the surfaces. The main physical barrier limiting the effectiveness and velocity of transportation of liquids and in liquids lies in the fact that the fluid or systems operating in fluids have to overcome significant resistance accompanying the movement of the systems relative to the fluid or the movement of liquids transported through pipelines. The aero- and hydrodynamic resistance increases in proportion to the cube of relative velocity of the object and the fluid. One of the many ways proposed to reduce flow resistance of liquids is intentional modification of physicochemical and geometric properties of surfaces in contact with a flowing liquid.
There are also natural organisms, like water striders that, utilize surface tension to walk on the surface of water. Water striders for example, have long thin legs which show water contact angles of around 167°, allowing them to stand on the surface of water using surface tension forces alone. The amount of weight that surface tension can support before the object penetrates the surface is proportional to the perimeter of material. There have been attempts to mimic the behavior of water striders by making objects fitted with long thin wires coated with hydrophobic material like fluorine compounds. Even though using such materials enabled the synthesis of systems which could stay statically on surface of water, the method lacked miniaturization due to the use of long wires to increase the perimeter. It would be desirable to provide the ability to produce miniature floating devices. Such devices may need forces beyond the realm of buoyancy to hold them on the surface of water. Conventional devices, utilizing buoyant force require displacement of water mass equivalent to the mass of the floating object. Such a system may fail if the density of the object is greater than that of water. Surface tension forces may be utilized in such cases. These forces depend on hydrophobicity of material. Hydrophobic surfaces resist penetrating the surface of water. The amount of resistance offered to penetration depends on hydrophobicity of the material.
Attempts have been made to reduce the fluid friction resistance accompanying relative movement of a liquid or fluid relative to a solid surface. Attempts have been made to produce hydrophobic surfaces which repel water or other liquids very effectively. Hydrophobic surfaces (e.g. ultra-hydrophobic surfaces and superhydrophobic surfaces) are used in many technological applications. Hydrophobic surfaces can reduce and/or minimize frictional drag in water, minimize corrosion of an underlying material, and serve as self-cleaning surfaces. Some hydrophobic surfaces (e.g. ultra-hydrophobic surfaces and superhydrophobic surfaces) have surface energy attributes and/or morphology attributes (e.g. fine surface roughness) that provide for relatively strong water repellency. However, adequate morphology attributes are difficult and costly to produce, and can be difficult, impractical, and/or impossible to implement on a large scale. Known hydrophobic surface configurations also are either impractical and/or impossible to implement in some desirable applications.
Such attempts have used organic materials such as polymeric materials wherein techniques such as etching, sputtering, lithographic techniques, film deposition from solution, electrolytic deposition or other techniques. While such methods have shown capability for creating a rough surface on particular materials, the methods are fairly limited in application and also require expensive and complicated processing techniques. Further, such attempts have not been useful for many applications, as the organic materials used do not have sufficient thermal stability, electrical conductivity or other attributes desired for many applications. It would be desirable to provide a hydrophobic or superhydrophobic surface configurations and methods which overcome such limitations.
Thus, there exists a need for improved hydrophobic or superhydrophobic surfaces or surface coatings, and techniques of forming hydrophobic or superhydrophobic surfaces, where the hydrophobic characteristics of the surface have a long life span, and the surfaces or coatings can be formed in a repeatable and cost effective manner, particularly in association with metallic surfaces and materials.